According to Paul William in his book (Mahayana: Doctrinal Foundations) the Sambhogakaya is an impermanent body so therefore it can only teach high level Bodhisattvas for a limited time. Same thing with the Nirmanakaya form,it is also impermanent.
What is your view on this? What is the point of becoming a Buddha if one can only help sentient beings for a limited time? You'll free yourself sure but what about everyone else
Reality is impermanent, always. It can't be avoided or reduced. It's one of the three marks of existence; impermanence, unsatisfactoriness, and emptiness. Becoming a sammasumbuddha will not make existence permanent, satisfactory, or occupied by self. From the get-go the teaching of the enjoyment body of the Buddha has regarded it as momentary. The emanations bodies are of course also momentary because they emanate from the enjoyment body. The purpose of becoming a Buddha is not lost, however. You aren't helping beings for a limited time, you're helping all who you teach and everyone in your field infinitely. The entire point is that your help is infinite and not limited, that's what Supreme Enlightenment means. Samboghakaya and nirmanakaya are just conventions to describe the miraculous virtue of Supreme Buddhas. By their virtue an enjoyment body emerges and emanates transformation bodies. But it is strictly conventional. The dharmakaya does not really go away. It's everpresent and everlasting. It presupposes Buddhas and it is uplifted by the Buddhas. It is a symbol of supreme virtue, and from an advanced perspective (such as Mahayana) it is the source of Bodhisattvas (here referring to Enlightenment Beings as like representations of buddhanature).